1. Field of the Invention
The present invention relates to an image formation apparatus using an electrophotographic process and a process cartridge for an image formation apparatus.
2. Description of the Related Art
Conventionally, an image formation apparatus using an electrophotographic process has a charging device for charging the surface of a photoconductor as a body to be charged. One type of charging process used in the charging device is a charging process based on proximity discharge. In this process, the photoconductor surface is charged due to the proximity discharge by contacting a charging member with the photoconductor surface or arranging a charging member close to the photoconductor surface without contact.
Recently, the attainment of high quality image and miniaturization of the apparatus has been increasingly desired and the attainment of high quality image and miniaturization of the charging device has been problematic. Against such a problem, a charging device using a proximity discharge process in which a charging member in contact with or proximity to a photoconductor is employed is useful since no large charging device is needed.
However, it is found that the photoconductor surface is deteriorated in the charging process due to the proximity discharge, since the discharge concentrates in proximity to the photoconductor surface. The deterioration of the photoconductor surface caused by the proximity discharge is different from the case of mechanical friction and occurs also when a member contacting to the photoconductor is not used.
FIG. 1 is the result of a measurement with respect to the change of the film thickness of a photoconductor surface when only a charging member was arranged in proximity to the photoconductor surface but did not contact it and charging tests were performed continuously for approximately 150 hours, in order to investigate the degree of deterioration of the photoconductor surface caused by the proximity discharge.
The photoconductor used herein was an organic photoconductor that contains polycarbonate as a binder resin in the charge transportation layer of the surface thereof. Also, all members contacting the photoconductor were removed and charging was carried out using a non-contact charging roller to which a voltage with an AC bias superposed to a DC bias was applied. As a result we found the fact that ground film quantity of the photoconductor surface gradually increased and the film thickness of the photoconductor gradually decreased. The mechanism of the decrease of the film thickness has not been clear and has been under consideration until now. However, as the photoconductor with the reduced film thickness was analyzed, a carboxylic acid was detected whereby it is considered that the polycarbonate composing the photoconductor was decomposed. Thus, since a material was detected whereby it is considered that a component composing the photoconductor is decomposed by the proximity discharge, the reduction mechanism of the film thickness of the photoconductor is considered to be as follows.
FIG. 2A is a diagram that illustrates an example of the state of a photoconductor surface when the surface of the photoconductor 1 is deteriorated by proximity discharge and FIG. 2B is a diagram that illustrates an example of such a state that a charging roller 2a opposes a photoconductor surface via a narrow gap.
As the proximity discharge is caused, the energy of particles (ozone, an electron, excited molecules, ions, plasma, and the like) generated by the discharge is applied to a charge transportation layer 1a of the photoconductor surface in a discharge area on the photoconductor surface. The energy resonates a bonding energy of a molecule composing the photoconductor surface and is absorbed. As shown in FIG. 2A, in the charge transportation layer 1a, a chemical deterioration is caused such as the decrease of a molecular weight by cutting a chain of a resin molecule, the decrease of the entanglement of the chains of the polymer molecules, and evaporation of the resin. It is considered that the film thickness of the charge transportation layer 1a of the photoconductor surface gradually decreases by such a chemical deterioration of the photoconductor caused by the proximity discharge. In such a situation, when mechanical friction is applied to the photoconductor surface using a cleaning blade, the abrasion of the photoconductor is further accelerated.
Thus, it is found that a countermeasure to the reduction of the film thickness by the chemical deterioration of the photoconductor surface caused by the proximity discharge is needed beside a countermeasure to the reduction of the film thickness caused by the mechanical friction, which has been taken conventionally. Herein, it is considered that since the aforementioned reduction of the film thickness of the photoconductor surface caused by the proximity discharge occurs due to the energy of the particles generated by the discharge, the reduction does not only occur in the case of using polycarbonate but also occurs in the case of using a photoconductor made of another material.
Conventionally employed countermeasures to prevent the film thickness of the photoconductor surface from decreasing are as follows. For example, a photoconductor surface is coated with amorphous silicon carbide to improve an abrasive resistance. Also, for example, Japanese Laid-Open Patent Applications No. 2002-207308 and No. 2002-229227 disclose that an inorganic material such as alumina is dispersed in a charge transportation layer (CTL) being a surface layer of an organic photoconductor so as to improve an abrasive resistance of the photoconductor. However, such a structure can improve the resistance to a mechanical abrasion but cannot prevent the chemical deterioration of the photoconductor surface caused by the proximity discharge. In this case, since the photoconductor surface becomes difficult to be refaced by improving the mechanical abrasive resistance, a matter of the photoconductor surface, which is deteriorated by the proximity discharge, becomes easy to remain and it causes the generation of an image deletion or each kind of an image defect.
Recently, proximity discharge prevails in a charging device used in an image formation apparatus and the influence of chemical deterioration caused by the proximity discharge cannot be avoided. Therefore, the aforementioned method for improving the mechanical abrasive resistance of a photoconductor causes the elimination of a deteriorated matter on a photoconductor surface to be difficult and accelerates the generation of an image defect, and, in fact, the attainment of long life of the photoconductor has not been achieved yet. Furthermore, it is found that taking only the improvement of the mechanical abrasive resistance of the photoconductor reduces the resistance of a cleaning blade and inadequate cleaning or the generation of filming tends to be induced. Therefore, in order to realize the long life of a photoconductor and an image formation apparatus using it, while the abrasive resistance of the photoconductor is enhanced, the stability of an image has to be balanced with it. Accordingly, a method for not only improving the mechanical abrasive resistance of a photoconductor but also suppressing the chemical deterioration of the photoconductor caused by proximity discharge has been strongly desired.
Japanese Laid-Open Patent applications No. 2002-055580 and No. 2002-244487 disclose image formation apparatuses with a device for applying zinc stearate on the surface of an image supporter. These methods are similar to a discharge deterioration prevention means described below with respect to the present invention and the objects of applying zinc stearate in these methods are to lower a friction coefficient of a photoconductor surface in order to prevent inadequate cleaning on the photoconductor surface.
Additionally, Japanese Laid-Open Patent Applications No. 2002-244516 and No. 2002-156877 similarly disclose image formation apparatuses with a device for applying zinc stearate on a photoconductor surface. The objects of these techniques are to suppress fusion or filming of a developer caused by activating the photoconductor surface with discharge and, therefore, zinc stearate is applied.